Mobility support for Internet devices is quite important, since mobile computing is getting more widespread. It is expected that the number of mobile computers will increase dramatically. Furthermore, there are already first products of cellular phones offering IP (Internet Protocol) services based on WAP (Wireless Application Protocol) or GPRS (General Packet Radio Services), and their number will increase rapidly. Cellular devices of the 3rd generation will be packet switched devices instead of circuit switched devices. Therefore, IP services on 3rd generation cellular devices will be an integral part in the future.
Today, several problems have to be faced making roaming with mobile Internet devices difficult. The problems start if somebody disconnects his mobile device from the Internet in order to connect it elsewhere. Normally, he would not be able to continue communication until he configures the system with a new IP address, the correct net mask and a new default router.
This problem is based on the routing mechanisms which are used in the Internet. IP addresses define a kind of topological relation between the linked computers. Today's versions of Internet protocols assume implicitly that a node has always the same point of attachment to the Internet. Additionally, the node's IP address identifies the link on which the node resides. If a node moves without changing its IP address, there is no information in its network address about the new point of attachment to the Internet.
To support mobile devices, which dynamically change their access points to the Internet, the Internet Engineering Task Force (IETF) currently standardizes a protocol supporting mobile Internet devices, called Mobile IP. There are two variations of Mobile IP, namely Mobile IPv4, based on IPv4 (Internet Protocol version 4), and Mobile IPv6, based on IPv6 (Internet Protocol version 6). The latter one is described in David W. Johnson and Charles Perkins, Mobility Support in IPv6, Internet Draft, 2000. Further information on the IPv6 can be obtained from IETF specification RFC 2460, 1998.
Mobile IPv6 allows an IPv6 host to leave its home subnet while transparently maintaining all of its present connections and remaining reachable to the rest of the Internet. This is realized in Mobile IPv6 by identifying each node by its static home address, regardless of its current point of attachment to the Internet. While a mobile node is away from home it sends information about its current location to a home agent on its home link. The home agent intercepts packets addressed to the mobile node and tunnels them to the mobile node's present location. This mechanism is completely transparent for all layers above IP, e.g. for TCP (Transmit Control Protocol), UDP (User Datagram Protocol) and of course for all applications. Therefore, domain name server (DNS) entries for a mobile node refer to its home address and don't change if the mobile node changes its Internet access point. In fact, Mobile IPv6 influences the routing of packets but is independent of the routing protocol itself.
The solution given by Mobile IPv6 consists of creating a so-called care-of-address whenever a node changes its point of attachment to the web. The care-of-address is an IP address associated with a mobile node while visiting a foreign link. The subnet prefix of this IP address is a foreign subnet prefix. Among the multiple care-of-addresses that a mobile node may have at a time (e.g. with different subnet pre-fixes), the one registered with the mobile nodes home agent is called its “primary” care-of-address. A care-of-address' can be derived from the receipt of router advertisements in a so-called “stateless address auto-configuration” as described in S. Thomson and T. Narten, “IPv6 Stateless Address Auto-Configuration”, IETF specification RFC 2462, 1998, or can be assigned by a DHCP (Dynamic Host Configuration Protocol) server in a so-called “stateful address auto-configuration”. Mobile nodes are always identified by their (static) home address regardless of their current point of attachment to the Internet. While away from home each mobile node has an additional (temporary) address which identifies its current location. Thus, basically messages that arrive at the original home address are redirected or tunneled to the care-of-address.
Mobile IPv6 requires the exchange of additional information. All new messages used in Mobile IPv6 are defined as IPv6 destination options. These options are used in IPv6 to carry additional information that needs to be examined only by a packet destination node. In particular, a binding update option is used by a mobile node to inform its home agent or any other correspondent node about its current care-of-address. A binding acknowledgement option is used to acknowledge the receipt of a binding update, if an acknowledgement was requested. Furthermore, a binding request option is used by any node to request a mobile node to send a binding update with the current care-of-address. Finally, a home address option is used in a packet sent by a mobile node to inform the receiver of this packet about the mobile node's home address. If a packet with the home address option is authenticated then the home address option must also be covered by this authentication.
Due to the asymmetric nature of communications in the IP networks, the use of unidirectional links provides a way to improve network performance and capacity. Unidirectional links might be used to deliver downlink data, but the problem is that this option is not supported natively in IP networks.